1. Field of the Invention
This invention relates to electrical switchgear and particularly to switchgear adapted to have a dummy element mounted in it in place of a circuit breaker.
2. Background Information
Circuit breakers for medium voltage switchgear apparatus applications are generally housed in metal enclosures and are removable. The apparatus typically includes at least one levering-in mechanism or device to move a circuit breaker between a disconnect position, in which the primary contacts of the circuit breaker are fully disengaged from the mating primary contacts within the enclosure, and a connect position, in which the primary contacts of the circuit breaker and enclosure are fully engaged. Levering-in and levering-out/withdrawing of a circuit breaker is typically accomplished by using a detachable hand crank which is inserted into the levering-in mechanism to move the circuit breaker within the cell as a function of turning the crank.
Typically, power is provided to auxiliary devices and control circuitry through mating secondary contacts mounted with the circuit breaker in the enclosure. At some point during movement of the circuit breaker from the disconnect to the connect position, the respective secondary contacts must be engaged so the power is provided to the auxiliary devices and control electronics. When the secondary contacts are engaged, but the primary contacts are disengaged or disconnected, the auxiliary functions of the circuit breaker can be safely tested since the breaker is dead.
Dummy elements are used as a means of isolating circuits or bus sections in switchgear where operation is infrequent and a circuit breaker cannot be economically justified. The device consists of a circuit breaker mechanism frame and primary insulator supports with primary studs including disconnecting devices. Copper rods are secured in the location normally occupied by vacuum interrupters. The stationary structure is the same as for a circuit breaker. When the device is fully levered in, it connects the breaker primary disconnects to the enclosure primary contacts. The dummy element is not a switching device and therefore it requires a special interlocking system for safe operation. An interlock system is required that insures that the dummy element cannot be levered from the test position or the disconnect position to the connect position or vice versa, unless all sources to it are disconnected and locked in that position.
In a typical system, a dummy element is connected to an upstream or upline of a circuit breaker so power for the dummy circuit comes from or through the upstream circuit breaker. Both breaker compartments (breaker and dummy) contain positional interlocks and key interlocks that operate with the same key. The key is required to move the dummy element between the test and connect positions. The key can be removed from the upstream breaker circuit only when the breaker is locked in a de-energized (test/disconnect/withdrawn) position.
A typical interlocking system consists of an operator on the dummy element and a dummy/breaker position interlock in the dummy element's compartment as well as in all source breaker compartments connecting to the dummy element. In order to lever the dummy element, access to the compartment levering device is required. This access is blocked when the source circuit breaker(s) is in the connect position, regardless of whether the dummy element is in the test/disconnected position or the connected position. As stated above, the key to unlock the access can be obtained only by levering the source breaker(s) out to the test position and locking it (them) in that position. This assures that the dummy element can be levered in or out only in safe mode; i.e., when the source breaker(s) is locked in a test/disconnect or withdrawn position. The source breakers do not have the operator, so they are lockable only in the test position.
In case there is more than one source, the key for the dummy element interlock will usually be obtained from a transfer lock at which all the source keys are accumulated and captured before the dummy element key can be obtained. When applying the dummy element in the system, the position interlocks must be used in the dummy compartment as well as in all source breaker compartments.
The cell positional interlock for a breaker/dummy element is a very critical safety feature that prevents anyone from levering a dummy element, on or off, the bus stabs without the load being removed. If the dummy element is levered on or off the bus stabs with the load connected, severe arcing of the primary stabs and/or potential explosion could result. The interlock permits the dummy element to be key interlocked, such that the dummy element cannot be moved from the disconnected to connected or connected to disconnected/test positions without the source breaker or switch being opened, withdrawn to the disconnected position and key locked in that position.
There is a need for an improved interlock for switchgear containing a dummy element which cannot be easily defeated.
There is a need for such an improved interlock that can be retrofitted into existing switchgear.